essentially the trap is a half open cylinder that flips over rapidly dumping creature into bucket. Sort of a tumbling funhouse barrel. The advantage of this design is the minimal load on the servo even with a good sized creature chowing down in the trough.
The digital sensor is a Sharp GP2Y0D805Z0F from Pololu that I have I have watched closely and it's output seems very consistent. The primary concern is assuring a consistent delay prior to activating servo. Since I added counter that consistency has diminished. So I'm thinking of driving a couple of seven segment LEDs as opposed to pulsing count output.